1. Field of the Invention
The present invention relates to a reception device, a demodulator, and a communication method.
2. Description of the Related Art
In recent years, a multi-level modulation system has been considered as one means of increasing transmission capacity in mobile communication. In other words, in mobile communication, using a multi-level quadrature amplitude modulation (QAM), such as 16 QAM and 64 QAM processing many bits in a single symbol has been considered, instead of using a modulation such as quadrature phase shift keying (QPSK) for a conventional personal digital cellular telecommunications system (PDC) and a personal handy-phone system (PHS).
With QPSK, demodulation should be performed considering only phase variation occurring during transfer of a signal through a propagation path; however, with a multi-level quadrature amplitude modulation such as 16 QAM or 64 QAM, demodulation must be performed considering amplitude variation as well as phase variation. But, in mobile communication, sharp amplitude variation generally occurs, due to restriction and control of transmission power and fading. As a result, amplitude variation must be accurately estimated in order to use multi-level quadrature amplitude modulation for mobile communication.
Conventionally, amplitudes have been estimated using a pilot symbol when performing multi-level quadrature amplitude modulation (e.g., Japanese Patent Application Laid-Open No. 2002-217862). More specifically, a transmission side notifies a reception side of transmission power ratios of a pilot symbol to data symbols in order to perform channel estimation. The reception side then estimates the amplitudes of the data symbols based on the amplitude of the received pilot symbol and the transmission power ratios of the pilot symbol to the data symbols. The reception side also performs a hard decision for the received data symbols, based on the estimated data symbol amplitudes. Alternatively, other than notifying of transmission power ratios by the transmission side, there has been a method of estimating the amplitude, whereby the reception side estimates transmission power ratios and estimates the amplitude using the estimated transmission power ratios. In addition, there has been a method of estimating the amplitude, which is using the received power of a data symbols and the noise dispersion estimated from the pilot symbol (e.g., ‘PILOT POWER RATIO SIGNALING (Corrected)’, Motorola, TSG-RAN-WG1 HSDPA Ad-Hoc. TSGR1 (01) 1087, Sophia Antipolis, France, Nov. 5 to 7, 2001).
However, the conventional amplitude estimation method needs a transmission side to notify a reception side of the transmission power ratios of a pilot symbol to data symbols. The reception side must receive the notification of the transmission power ratios and then estimate amplitudes using the notified transmission power ratios. Furthermore, the notification may contain an error. Moreover, in the case where the reception side estimates transmission power ratios, to begin with it must estimate transmission power ratios and then estimate amplitudes using the estimated ratios. Also, in order to estimate amplitudes using the received power of data symbols and the noise dispersion estimated from a pilot symbol, to begin with estimation of noise dispersion and then estimation of amplitudes using the estimated noise variance is required. Therefore, the conventional amplitude estimation method causes a control delay, an increase of control load on both a transmission and a reception side, and difficulty in accurate estimation.